This invention relates in general to the field of microwave circuits, in particular to microwave divider/combiner circuits that divide/combine received signals of differing frequency, and more particularly to antenna systems that receive Global Positioning System (GPS) L1 and L2 frequency signals.
Microwave power dividers and combiners have been used in a wide variety of applications for many years and in their most basic form are three port devices. In the case of a power divider, one port is often referred to as an input port and the other ports are often referred to as the output ports. In the case of a power combiner, one port is often referred to as an output port and the other ports are often referred to as the input ports. Passive microwave power dividers and combiners generally operate as either a power combiner or power divider, and therefore whether the ports are referred to as input or output ports is interchangeable. In many applications, power divider/combiners operate as both a combiner and a divider, for example, when used in a beamforming network for a phased array antenna that operates as both a transmit and receive antenna.
Microwave power dividers and combiners may use microwave transmission lines such as stripline transmission lines or microstrip transmission lines. A microwave stripline transmission line is comprised of three conductors wherein a center conductor is provided between two layers of dielectric material which may lie between two ground-plane conductors. A microstrip transmission line on the other hand often has a conductor fabricated on a layer of dielectric material and a ground plane conductor on an opposite side of the dielectric material.
In many microwave signal applications it is desirable to be able to split microwave signals into one or more signals. A signal divider often takes the form of a distributed quarter-wavelength section of transmission line in a xe2x80x9cTeexe2x80x9d configuration. The signal power is split into two components; one output at each of two output ports. In addition to splitting microwave signals, it is frequently desirable to be able to combine two microwave frequencies to a single port. For example, two antenna inputs may be combined to provide a single input to a receiver. Like signal dividers, combiners often employ two or more quarter-wavelength sections coupled together at a common junction to combine two microwave signals.
A problem with conventional signal dividers and signal combiners and especially dividers and combiners that utilize quarter-wavelength sections is their inability to efficiently combine and/or divide signals of different frequencies. For example, when two antennas receive separate frequencies that need to be combined into a single receiver input, combining the signals can result in up to a 50% loss in received power from each signal because the signal power is split between the receiver input and the output from the other antenna. This not only reduces receiver performance, but may result in radiation of the received signals through the other antenna. Receiver performance is especially important to systems that utilize timing measurements for the basis of determining position. For example, receivers in advanced missile position determining systems may acquire and track signals provided the Global Positioning System (GPS) system satellites.
Accordingly, there is a general need for a method and apparatus that provides for improved position determination in missile systems. There is also a general need for a method and apparatus that provides for improved receiver performance. There is also a general need for a method and apparatus that provides improved signal strength of received signals to a receiver. There is also a general need for a method and apparatus that reduces radiation of received signals by other antennas. There is also a general need for a signal combiner and method of combining signals that more efficiently combines signals of different frequencies. There is also a general need for a method and apparatus for dividing signals and more efficiently separating signals of different frequencies. There is also a general need for divider/combiner structures for use with signals of different frequencies.
The needs in the art are addressed by the present invention which, in one embodiment, provides a signal divider/combiner for dividing/combining signals of a first and second frequency. In this one embodiment, the signal divider/combiner includes first, second and third transmission lines meeting at a junction, a first transmission line stub section coupled to the first transmission line to block signals of the second frequency along the first transmission line, and a second transmission line stub section coupled to the second transmission line to block signals of the first frequency along the second transmission line. In this embodiment, the first transmission line stub section is a first open-circuit stub positioned a first distance from the junction. The first distance is substantially equal to a quarter-wavelength of the second frequency. The second transmission line stub section is a second open-circuit stub positioned a second distance from the junction. The second distance is substantially equal to a quarter-wavelength of the first frequency. In this embodiment, the first, second and third transmission lines and the first and second open-circuit stubs are microstrip transmission lines having substantially the same impedance, and the first frequency is a Global Positioning System (GPS) L1 frequency, and the second frequency is a GPS L2 frequency.